Barbara Walker says to cast on with color A and purl one row.
Rows 1&2 and 5&6 are worked with B, rows 3&4 and 7&8 are worked with A.
When binding off on row 8, leave out the yarnovers to prevent spread.
(I think this should look good as a single color pattern, too - gannet)
Barbara Walker, A Second Treasury of Knitting Patterns, p.97